1. Field of the Invention
The present invention is generally related to the positioning of work pieces and, in particular, to the positioning of microtiter plates in a liquid handling system.
2. Description of the Related Art
A wide variety of positioning systems are employed to bring vessels of various sorts into a desired position, where the vessels may receive various reactants, reagents, solvents or other fluids, for example. In particular, positioning systems are widely employed in the field of chemical synthesis and analysis. Plates which contain a multiplicity of small wells or vessels, often referred to as microtiter plates, are positioned under multi-tip dispensers to receive fluids from the dispensers, with each well in the plate receiving fluid from one dispenser tip at a time. Positioning systems range from the very complex, which require sophisticated computer control systems coupled to a complex series of actuators or robotic systems, to relatively simple positive displacement systems which move a work piece, vessel, or similar item against a rigid stop.
The more complex systems provide the advantage of greater placement flexibility. In the field of chemical synthesis and analysis, such flexibility may allow a robot arm with an attached pipette to extract a sample from one of several reservoirs, transport the sample to any one of hundreds, or even thousands, of reaction wells located within a microtiter plate, and release the sample into the well. However convenient such a system may be, the complexity of such a system leads to greater expense. Furthermore, since the robot arm, or other pipette-holding mechanism, must be carefully accelerated and decelerated many times over, there are many opportunities for mispositioning the pipette. Additionally, such a positioner's movements are typically comparatively slow, since the positioner must be carefully accelerated and decelerated in order to position the pipette properly. The complexity of such a positioning system also introduces the possibility of multiple failure mechanisms. Therefore, reliability and maintenance can be major issues with such a positioning system.
On the other hand, positive displacement positioning systems, which displace a work piece against a rigid stop, tend to be much simpler, much less expensive, and more reliable than a complex positioner such as the ones just described. Nevertheless, while positive displacement, or rigid stop, positioners typically provide highly accurate positioning, the typical rigid stop positioner provides a maximum of only four discrete positions, which may be described as top-left, top-right, bottom-left and bottom-right, that is, four positions defined by two rigid stops in either of two orthogonal axes. Such a positioner may be employed, for example, to translate a 384 well microtiter plate under a standard 96-tip dispenser and thereby allow the 96 tips of the dispenser to engage with one of four distinct 96-member sets of wells within the microtiter plate. Although the simplicity, reliability, and relative low cost of rigid stop positioners make them highly desirable, their limited positioning capabilities severely restrict their application.
Many applications, particularly in the field of combinatorial chemical synthesis, would benefit from the ability to position work pieces such as microtiter plates in more than just four discrete positions. For example, microwell plates having 1,536 wells are available for use. When using a currently widely used 96-tip dispenser, a minimum of sixteen positions would be required to access all the wells within such a plate. At the same time, it would be highly desirable to retain the simplicity, reliability and low cost that a rigid stop positioning system can offer.